Recently, white LEDs are actively under developments and applications thereof includes illumination, vehicle mounted lamps and liquid crystal backlights. The white LED has a characteristic of significantly high on/off switching response speed when compared to other white light sources such as a fluorescent lamp. For this reason, a visible light communication system which enables an illumination light of the white LED to have a data transmission function, i.e., the white LED light as a data transmission medium has been proposed. That is, a transmitter of the visible light communication system modulates a luminescent intensity of the white LED according to a transmission data and the receiver converts the intensity of the light into an electric signal through a photoelectric converter such as a photo diode (referred to as a “PD” hereinafter) in order to transmit data.
For example, the white LED may be classified into three types according to luminescence schemes thereof.
(1) Blue-Light-Excited-Type White LED
This LED combines a blue LED with a fluorescent material that mainly emits a yellow light. For example, an yttrium aluminum garnet (YAG) group-based fluorescent material is arranged around the blue LED, which is accommodated in a single package. In this type of LED, the surrounding fluorescent material is excited by a blue light outputted from the blue LED disposed at the center and the light (mainly yellow) that is mainly complementary to blue is outputted from the fluorescent material. By mixing yellow fluorescence from the fluorescent material and the blue light from the blue LED, a pseudo-white light is obtained.
The blue-light-excited-type white LED has the following advantages: a) it has high energy efficiency and high luminous intensity when compared to other types and b) due to a simple construction thereof, it can be manufactured at a low cost. On the other hand, it is disadvantageous in that it has poor color rendering. Color rendering refers to characteristics of color appearance of an object under illumination and the closer a color is to that perceived under natural light, the better the color rendering.
(2) Ultraviolet-Light-Excited-Type White LED
This LED combines ultraviolet light with fluorescent materials that emit lights of three primary colors of red (R), green (G) and blue (B), respectively. The fluorescent materials that emit three primary colors of R, G and B are arranged around an ultraviolet LED, which is accommodated in a single package. In this type of LED, the surrounding fluorescent materials are excited by the ultraviolet light outputted from the ultraviolet LED disposed at the center and the lights of the three primary colors of R, G and B are outputted from the fluorescent materials, respectively. By mixing the R, G and B light, a white light can be obtained.
The ultraviolet-light-excited-type white LED is advantageous in that the LED has superior the color rendering . On the other hand, the LED is disadvantageous in that a) it has low energy efficiency and poor luminous intensity when compared to the blue-light-excited-type white LED, and b) a driving voltage of the LED is high due to ultraviolet luminescence.
(3) Three-Color-Emitting-Type White LED
This LED combines three types of LEDs, namely R, G and B LEDs. The three types of LEDs, i.e., the red LED, the green LED and the blue LED, are accommodated in a single package. In this type of LED, a white light is obtained by simultaneously causing the LEDs to emit the three primary colors, respectively.
The three-color-emitting-type white LED is advantageous in that the LED has a superior color rendering similar to the ultraviolet-light-excited-type white LED. On the other hand, it is disadvantageous in the white LED requires high manufacturing cost due to the three types of LEDs accommodated in a single package when compared to other types of LEDs.
A conventional optical communication apparatus employing a white LED is illustrated in FIG. 13A. In the optical communication apparatus shown in FIG. 13A, when transmission data is provided to a driver 902 of a transmitter 900, the corresponding driving current is outputted to a white LED 904 and the white LED 904 emits light. For example, the white LED 904 blinks when modulated by OOK (on-off keying) for example. A light signal outputted from the white LED 904 is incident on a PD 912 of a receiver 910. The light signal is converted into a current signal by the PD 912 and the current signal is converted into a voltage signal by a trans-impedance amplifier (current-voltage conversion amplifier) 914. The voltage signal is subjected to desired equalization processing by an equalizer 916 and then is binarized by a limiting amplifier 918 to be outputted as received data.
When the blue-light-excited-type white LED is employed as the white LED 904, the response speed of the light outputted from the fluorescent material is low and thus only a transmission speed of about several Mbps at most can be obtained. In order to overcome the drawback, a method wherein an LED light transmission color filter, through which only the blue light passes, is installed in front of a photoelectric converter to remove an optical component having a low response speed which is outputted from the fluorescent material has been proposed to achieve a high speed. FIG. 13B shows a configuration of such device wherein a blue color filter 922 is arranged on a light-incident side of the PD 912 of the receiver 920. Through the blue color filter 922, the light in the optical signal emitted from the fluorescent material having the low response speed is removed. As a result, only the light of the blue LED is incident on the PD 912 thereby achieving, data transmission faster than that of the above-described configuration. However, even using this method, only a transmission speed of about tens of Mbps can be obtained at most.
In addition, similar to the blue-light-excited-type white LED, a transmission speed is only several Mbps when the ultraviolet-light-excited-type white LED employed as the white LED 904. Further, the driving voltage of the LED is increased resulting in difficulty in designing a driving circuit. A method of obtaining an increase in the response speed of the light emitted from the fluorescent material through improvement of the fluorescent material is in research. However, problems that a desired luminous intensity is not obtained and a high cost of the fluorescent material itself is increased have not yet been solved.
In addition, when the above-described three-color-emitting-type white LED is employed as the white LED 904, there is no fluorescent component compared to other LEDs and it is possible to transmit data by performing wavelength multiplexing whereby the respective LEDs carry different signals to achieve high speed transmission. However, since a plurality of LEDs are used, the cost increases.
As described, high-speed transmission can be expected from a general-purpose cost-advantageous blue-light-excited-type white LED. As an improvement from this viewpoint, the optical communication system illustrated in FIG. 13C includes a transmitter 930 with a peaking circuit 932. The transmitter 930 generates and adjusts driving current waveforms to obtain an optimum driving current waveform suitable for high-speed modulation. Accordingly, the transmitter 930 may output an optical signal suitable for high speed transmission even in sunlight and under a fluorescent lamp.
However, since an analog peaking circuit is used, overcurrent that exceeds a rated current of the LED may flow thereby damaging the LED. Further, since passive components such as resistors and condensers are used, it is difficult to perform adjustments for obtaining optimum driving conditions.